Atenolol interaction with aspirin, allopurinol, and ampicillin.

Atenolol kinetics were investigated in six healthy subjects after 100 mg orally, as monotherapy a 6-day treatment began 48 hr later. After a therapy-free interval of 4 wk, the same subjects received the same dose of atenolol with 1 gm ampicillin, 500 mg aspirin, and with 300 mg allopurinol. Allopurinol and aspirin did not substantially alter the kinetics of atenolol. After a single oral dose of 100 mg atenolol combined with 1 gm ampicillin, the bioavailability of atenolol was reduced to 36 +/- 5% compared to 60 +/- 8% after monotherapy. During long-term treatment with atenolol and ampicillin the bioavailability of atenolol fell to 24% (P less than 0.01). Mean peak plasma levels were lowered from 511 +/- 59 ng/ml on monotherapy to 344 +/- 33 ng/ml after the combination with ampicillin. The area under the plasma level-time curve, mean steady-state concentration, and urinary recovery were reduced, also. Twelve hours after 100 mg atenolol and 1 gm ampicillin, exercise tachycardia was significantly higher than after atenolol alone. During the 4-wk treatment in six hypertensive patients blood pressure levels of those on atenolol alone were not different from those on the combination therapy with ampicillin.